Light emitting element, method of manufacturing the same, and semiconductor device having light emitting element

ABSTRACT

An InGaN active layer is formed on a sapphire substrate. A p-side electrode is formed on the InGaN active layer to supply an electric current to this InGaN active layer. The p-side electrode includes {circle over (1)} an Ni layer for forming an ohmic contact with a p-GaN layer, {circle over (2)} an Mo layer having a barrier function of preventing diffusion of impurities, {circle over (3)} an Al layer as a high-reflection electrode, {circle over (4)} a Ti layer having a barrier function, and {circle over (5)} an Au layer for improving the contact with a submount on a lead frame. The p-side electrode having this five-layered structure realizes an ohmic contact and high reflectance at the same time.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Application No. 2000-200298, filed Jun.30, 2000, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a light emitting element and,more particularly, to the electrode structure of a light emittingelement.

[0004] 2. Description of the Related Art

[0005] The recent progress of light emitting elements is remarkable. Inparticular, small-sized, low-power-consumption, high-reliability lightemitting diodes (LEDs) are developed and extensively used as displaylight sources.

[0006] Red, orange, yellow, and green LEDs currently put to practicaluse are made of group III-V compound semiconductors using As and P asgroup V elements, e.g., AlGaAs, GaAlP, GaP, and InGaAlP. On the otherhand, green, blue, and ultraviolet LEDs are made of compoundsemiconductors such as GaN. In this way, LEDS having high emissionintensity are realized.

[0007] When the luminance of these LEDS is increased, applications suchas outdoor display devices and communication light sources arepresumably greatly extended.

[0008]FIG. 1 shows the structure of a conventional violet LED.

[0009] A light emitting element 110 for emitting violet light is bondedon a lead frame 120 by silver paste 130. The p- and n-electrodes of thislight emitting element 110 are connected to the lead frame 120 bybonding wires 150. The light emitting element 110 is covered with anepoxy resin 180.

[0010]FIG. 2 shows the light emitting element shown in FIG. 1.

[0011] On a sapphire (Al₂O₃) substrate 200, an n-GaN layer 210 and ap-GaN layer 220 are formed. The n-GaN layer 210 has a recess. Since thep-GaN layer 220 is not present on this recess, the n-GaN layer 210 isexposed in this recess of the n-GaN layer 210.

[0012] An n-side electrode 230 is formed on the recess of the n-GaNlayer 210. A transparent electrode 240 having properties of transmittinglight is formed on the p-GaN layer 220. In addition, a bonding electrode250 for wire bonding is formed on the p-GaN layer 220.

[0013] When a voltage is applied between the two lead frames 120 in theLED shown in FIGS. 1 and 2, an electric current is injected into thep-GaN layer 220 from the bonding electrode 250 and the transparentelectrode 240. This electric current flows from the p-GaN layer 220 tothe n-GaN layer 210.

[0014] In the boundary (p-n junction) between the p-GaN layer 220 andthe n-GaN layer 210, light having energy hν (h: Planck's constant,ν=c/λ, c: velocity of light, λ: wavelength) is generated when theelectric current flows. This light is emitted upward from thetransparent electrode 240.

[0015] In the transparent electrode 240, however, the lighttransmittance and the conductivity have a relationship of trade-off.

[0016] That is, to increase the light transmittance, the thickness ofthe electrode need only be decreased. However, if the electrodethickness is decreased, the conductivity lowers. When the conductivitylowers, no electric current can be supplied to the whole p-n junctionany longer, and this decreases the light generation efficiency. Also, toincrease the conductivity, the thickness of the electrode need only beincreased. However, if the electrode thickness is increased, the lighttransmittance lowers. When the light transmittance lowers, lightgenerated in the p-n junction cannot be efficiently extracted to theoutside of the chip.

[0017] As a technology by which this problem is solved, a technology ofemitting light toward the sapphire substrate 200 is known.

[0018]FIG. 3 shows a light emitting element using this technology.

[0019] Since this light emitting element is bonded on a lead frame byflip chip bonding, an LED having this light emitting element is called aflip chip type LED.

[0020] A high-reflectance electrode 260 is formed on p-GaN 220. Of lightgenerated in the p-n junction, light traveling to a sapphire substrate200 is directly emitted to the outside of the chip. Of light generatedin the p-n junction, light heading to the electrode 260 is reflected bythis electrode 260. The reflected light travels to the sapphiresubstrate 200 and is emitted to the outside of the chip.

[0021] The sapphire substrate 200 will be described below.

[0022] When InGaN is used as an active layer, an LED currently put topractical use emits light within the range of blue to green. The bandgapof the sapphire substrate 200 is approximately 3.39 eV (wavelength λ≈365nm) at room temperature (300 K). That is, the sapphire substrate 200 hasproperties of transmitting light within the range of blue to green (thewavelength λ is approximately 400 to 550 nm).

[0023] A flip chip type LED is very effective as a technology ofextracting light to the outside of the chip with high efficiency, butalso has a problem.

[0024] That is, it is generally difficult to form an ohmic contact withthe p-GaN 220 when the high-reflectance electrode 260 is used. Thisohmic contact is an essential technology to reduce the contactresistance between the electrode 260 and the p-GaN 220 and therebyimprove the performance of the element.

[0025] Conventionally, therefore, the electrode 260 is given atwo-layered structure including an ohmic layer for forming an ohmiccontact and a high-reflection layer having high reflectance. The ohmiclayer improves the performance and the high-reflection layer increasesthe light emission efficiency at the same time.

[0026] Unfortunately, the ohmic layer obtains an ohmic contact byinterdiffusion of metal atoms between this ohmic layer and the p-GaN220, so these metal atoms naturally diffuse from the ohmic layer to thehigh-reflection layer. Since this diffusion lowers the performance andreliability of the light emitting element, it must be eliminated.

[0027]FIG. 4 shows an LED made of group III-V compound semiconductorshaving As and P as group V elements.

[0028] This LED emits light within the range of red to green.

[0029] On an n-GaAs substrate 300, an n-GaAs buffer layer 310 and ann-InGaAlP cladding layer 320 are formed. On this n-InGaAlP claddinglayer 320, an InGaAlP active layer 330, a p-InGaAlP cladding layer 340,and a p-AlGaAs current diffusing layer 350 are formed.

[0030] On the p-AlGaAs current diffusing layer 350, a p-GaAs contactlayer 360 and a p-side electrode 370 are formed. An n-side electrode 380is formed on the back side of the n-GaAs substrate 300.

[0031] In a light emitting element made of group III-V compoundsemiconductors (e.g., GaAs, AlGaAS, and InGaAlP) having As and P asgroup V elements, a sufficiently thick current diffusing layer (theAlGaAs current diffusing layer 350) is formed on a p-semiconductor layerwithout forming any transparent electrode on a p-semiconductor layer(the InGaAlP cladding layer 340). This sufficiently thick currentdiffusing layer has a function of evenly injecting an electric currentinto the entire InGaAlP active layer 330. Since the AlGaAs currentdiffusing layer 350 increases the light generation efficiency in thevicinity of the active layer, satisfactory optical power can be assured.

[0032] In the light emitting element shown in FIG. 4, an electriccurrent given to the p-side electrode 370 is injected into the InGaAlPactive layer 330 via the p-AlGaAs current diffusing layer 350. Lightgenerated near the InGaAlP active layer 330 is emitted upward from thep-AlGaAs current diffusing layer 350 except for a region where thep-side electrode 370 exists.

[0033] The film thickness, however, of the current diffusing layer 350must be increased to well diffuse the electric current for the reasonexplained below. That is, if the film thickness is small, the electriccurrent is not diffused but injected only into the active layer 330immediately below the p-side electrode 370. Consequently, most of thelight generated near the active layer 330 is interrupted by the p-sideelectrode 370.

[0034] In the fabrication of an LED and an LD (Laser Diode), MO-CVD(Metal Organic-Chemical Vapor Deposition) or MBE (Molecular BeamEpitaxy) is often used as a crystal growth method. This is so becausethese methods can well control the film thickness in the formation of athin film and thereby can form a high-quality film.

[0035] Unfortunately, these methods have the problem that they areinappropriate to form sufficiently thick films. That is, when MO-CVD orMBE is used, a very long time is required to form the sufficiently thickcurrent diffusing layer 350 used in the light emitting element shown inFIG. 4. This worsens the productivity.

[0036] Additionally, in the light emitting element shown in FIG. 4, thelight generated in the InGaAlP active layer 330 is absorbed by then-GaAs substrate 300. This lowers the light extraction efficiency of thelight emitting element shown in FIG. 4.

[0037] As a method of solving this problem of light absorption by theGaAs substrate 300, it is possible to form a flip chip type LEDdescribed earlier. However, the GaAs substrate 300 is opaque.Accordingly, a device from which this GaAs substrate 300 is removed isprepared, and a transparent substrate which transmits light is bonded tothis device.

[0038]FIG. 5 shows a light emitting element using this technology.

[0039] On a p-GaP substrate 400, a p-InGaAlP adhesive layer 410 and ap-InGaAlP cladding layer 420 are formed. An InGaAlP active layer 430 isformed on the p-InGaAlP cladding layer 420. On this InGaAlP active layer430, an n-InGaAlP cladding layer 440 and an n-AlGaAs window layer 450are formed.

[0040] In addition, an electrode 460 having high reflectance and ann-side electrode 470 are formed on the AlGaAs window layer 450. A p-sideelectrode 480 is formed on the back side of the p-GaP substrate 400.

[0041] Note that the GaP substrate 400 has a bandgap of 2.26 eV (λ≈548nm) at room temperature and is transparent to red light.

[0042] With this arrangement, of light generated in the InGaAlP activelayer 430, light traveling to the p-GaP substrate 400 is directlyemitted to the outside of the chip. Also, of light generated in theInGaAlP active layer 430, light heading to the electrode 460 isreflected by this electrode 460 having high reflectance. This reflectedlight travels to the p-GaP substrate 400 and is emitted to the outsideof the chip.

[0043] In the electrode 460, however, it is difficult to achieve anohmic contact and high reflectance at the same time by the use of asingle material. Therefore, this electrode 460 is given a two-layeredstructure including an ohmic layer and high-reflection layer. In thiscase, as described previously, the interdiffusion of metals between theohmic layer and the high-reflection layer is a problem.

[0044]FIG. 6 shows a light emitting element using the technology ofbonding a GaP substrate to a device from which a GaAs substrate isremoved.

[0045] In this technology, light is reflected by the bonding surfacebetween a GaP substrate 400 and a p-side substrate 480 and extractedupward from an AlGaAs window layer 450.

[0046] Compared to the light emitting element shown in FIG. 5, thislight emitting element shown in FIG. 6 is characterized by having nohigh-reflectance electrode on the n-AlGaAs window layer 450. In thisstructure, however, an alloy layer produced in the boundary between thep-GaP substrate 400 and the p-side electrode 480 scatters and absorbslight. This makes effective extraction of light to the outside of thechip difficult.

[0047] As described above, light is extracted from the conventionallight emitting elements by the two methods: extraction from a lightemitting layer, and extraction from a substrate.

[0048] When, however, a transparent electrode for diffusing an electriccurrent is formed on the entire surface of a light emitting layer andlight is extracted from this light emitting layer, the trade-off betweenthe light transmittance and the conductivity is a problem. That is, ifthe thickness of the transparent electrode is decreased to increase thelight transmittance, the conductivity lowers; if the thickness of thetransparent electrode is increased to increase the conductivity, thelight transmittance lowers.

[0049] In a structure in which an n-side electrode is formed on aportion of a light emitting layer and a thick current diffusing layer isformed below this n-side electrode, if light is to be extracted from thelight emitting layer by reflecting it by a p-side electrode formed onthe back side of a GaP substrate, this light is scattered and absorbedby the bonding surface between the GaP substrate and the p-sideelectrode. This worsens the light extraction efficiency.

[0050] Also, in a structure in which an n-side electrode is formed on aportion of a light emitting layer and a thick current diffusing layer isformed below this n-side electrode, if light is to be extracted from thesubstrate by reflecting it by the light emitting layer, the n-sideelectrode on the light emitting layer must have high reflectance. Thishigh-reflectance n-side electrode can be realized by using a two-layeredstructure including an ohmic layer and high-reflection layer as anelectrode structure. In this case, however, the interdiffusion of metalsbetween the ohmic layer and the high-reflection layer is a problem.

BRIEF SUMMARY OF THE INVENTION

[0051] It is an object of the present invention to provide a lightemitting element electrode structure capable of simultaneously achievingan ohmic contact and high reflectance and preventing interdiffusion ofmetals, thereby improving the performance and reliability of the lightemitting element and lowering the operating voltage of the element. Itis another object of the present invention to suppress scattering andabsorption of light in an electrode portion of a light emitting element,thereby increasing the light emission efficiency.

[0052] A light emitting element of the present invention comprises asubstrate, a light emitting element formed on the substrate to emitlight, and a first electrode contacting the light emitting layer. Thisfirst electrode includes an ohmic layer in ohmic contact with the lightemitting layer, a first barrier layer formed on the ohmic layer toprevent diffusion of metal atoms, and a light reflecting layer formed onthe first barrier layer to reflect light.

[0053] Additional objects and advantages of the invention will be setforth in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and obtained bymeans of the instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0054] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate presently embodimentsof the invention, and together with the general description given aboveand the detailed description of the embodiments given below, serve toexplain the principles of the invention.

[0055]FIG. 1 is a view showing a conventional LED;

[0056]FIG. 2 is a view showing the first example of a conventional lightemitting element;

[0057]FIG. 3 is a view showing the second example of a conventionallight emitting element;

[0058]FIG. 4 is a view showing the third example of a conventional lightemitting element;

[0059]FIG. 5 is a view showing the fourth example of a conventionallight emitting element;

[0060]FIG. 6 is a view showing the fifth example of a conventional lightemitting element;

[0061]FIG. 7 is a view showing an LED of the present invention;

[0062]FIG. 8 is a view showing the first embodiment of a light emittingelement of the present invention;

[0063]FIG. 9 is a view showing one step of a manufacturing method of thepresent invention;

[0064]FIG. 10 is a view showing one step of the manufacturing method ofthe present invention;

[0065]FIG. 11 is a view showing one step of the manufacturing method ofthe present invention;

[0066]FIG. 12 is a view showing one step of the manufacturing method ofthe present invention;

[0067]FIG. 13 is a graph showing the relationship between the electriccurrent and optical output of the light emitting element shown in FIG.8;

[0068]FIG. 14 is a graph showing the relationship between the thicknessand reflectance of a reflecting layer of the light emitting elementshown in FIG. 8;

[0069]FIG. 15 is a graph showing the relationship between the thicknessof reflectance of an ohmic layer of the light emitting element shown inFIG. 8;

[0070]FIG. 16 is a view showing a modification of the light emittingelement shown in FIG. 8;

[0071]FIG. 17 is a view showing the second embodiment of the lightemitting element of the present invention;

[0072]FIG. 18 is a view showing a modification of the light emittingelement shown in FIG. 17;

[0073]FIG. 19 is a view showing the third embodiment of the lightemitting element of the present invention;

[0074]FIG. 20 is a view showing a modification of the light emittingelement shown in FIG. 19;

[0075]FIG. 21 is a view showing the fourth embodiment of the lightemitting element of the present invention;

[0076]FIG. 22 is a view showing one step of a manufacturing method ofthe present invention;

[0077]FIG. 23 is a view showing one step of the manufacturing method ofthe present invention;

[0078]FIG. 24 is a view showing one step of the manufacturing method ofthe present invention;

[0079]FIG. 25 is a view showing one step of the manufacturing method ofthe present invention;

[0080]FIG. 26 is a view showing a modification of the light emittingelement shown in FIG. 21;

[0081]FIG. 27 is a graph showing the relationship between the electriccurrent and optical output of the light emitting element shown in FIG.21;

[0082]FIG. 28 is a view showing the fifth embodiment of the lightemitting element of the present invention;

[0083]FIG. 29 is a view showing one step of a manufacturing method ofthe present invention;

[0084]FIG. 30 is a view showing one step of the manufacturing method ofthe present invention;

[0085]FIG. 31 is a view showing one step of the manufacturing method ofthe present invention;

[0086]FIG. 32 is a view showing one step of the manufacturing method ofthe present invention;

[0087]FIG. 33 is a view showing one step of the manufacturing method ofthe present invention; and

[0088]FIG. 34 is a view showing one step of the manufacturing method ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0089] Light emitting elements and semiconductor devices using theselight emitting elements according to the present invention will bedescribed below with reference to the accompanying drawings.

[0090]FIG. 7 shows a lamp type LED of the present invention.

[0091] A submount 13 is placed on a lead frame 12. This submount 13 ismade of, e.g., a silicon substrate. On the upper surface of the submount13, high-conductivity ohmic electrodes 14-1 and 14-2 having a thicknessof about 100 μm are formed.

[0092] The positions of these ohmic electrodes 14-1 and 14-2 match thepositions of electrodes of a light emitting element 11. The ohmicelectrodes 14-1 and 14-2 are physically separated from each other, andan insulating film 19 is formed only immediately below the ohmicelectrode 14-2. This ohmic electrode 14-2 is electrically connected tothe lead frame 12 by a bonding wire 15. The lower surface of thesubmount 13 is adhered to the lead frame 12 by a conductive paste 16.

[0093] The light emitting element 11 for emitting violet light is placedon the submount 13. This light emitting element 11 has p- and n-sideelectrodes. The light emitting element 11 is bonded on the submount 13by flip chip bonding by using AuSn 17. The light emitting element 11 iscovered with an epoxy resin 18.

[0094]FIG. 8 shows the light emitting element shown in FIG. 7.

[0095] An n-GaN layer 21 is formed on a sapphire substrate 20. On thisn-GaN layer 21, an InGaN active layer 22, a p-AlGaN cladding layer 23,and a p-GaN layer 24 are formed. In addition, the n-GaN layer 21 has arecess at the edge of the sapphire substrate 20. Since the InGaN activelayer 22, the p-AlGaN cladding layer 23, and the p-GaN layer 24 do notexist on this recess, the n-GaN layer 21 is exposed in this recess.

[0096] An n-side electrode 25 is formed on the n-GaN layer 21 in therecess. A p-side electrode 26 is formed on the p-GaN layer 24. Thesurfaces of the n-GaN layer 21, the InGaN active layer 22, the p-AlGaNcladding layer 23, and the p-GaN layer 24 are covered with an insulatingfilm 27, except for regions where the n-side electrode 25 and the p-sideelectrode 26 are formed.

[0097] The n-side electrode 25 has a four-layered structure. Thisfour-layered structure includes a Ti layer 28, an Al layer 29, a Tilayer 30, and an Au layer 31 in this order from the n-GaN layer 21. Thep-side electrode 26 has a five-layered structure. This five-layeredstructure includes an Ni layer 32, an Mo layer 33, an Al layer 34, a Tilayer 35, and an Au layer 36 in this order from the p-GaN layer 24.

[0098] The Ni layer 32 is an ohmic layer for achieving an ohmic contactwith the p-GaN layer 24. The thickness of this Ni layer 32 is set toabout 4 nm. The Mo layer 33 and the Ti layer 35 function as barrierlayers for preventing diffusion of impurities. The Al layer 34 reflectslight at high reflectance. The Au layer 36 functions as an overcoatelectrode for improving the contact with the submount 13.

[0099] As shown in FIG. 7, the light emitting element shown in FIG. 8 isbonded by flip chip bonding on the submount 13 with the back side of thesapphire substrate 20 facing up.

[0100] In this light emitting element and the LED using the element,when a voltage is applied between the two lead frames 12 an electriccurrent is injected into the InGaN active layer 22 from the p-sideelectrode 26. When this electric current is injected into the InGaNlayer 22, the InGaN active layer 22 emits light. This light generated bythe LED is spontaneous emission light different from induced emissionlight. Hence, the light generated by the InGaN layer 22 has nodirectivity and is radiated in every direction from the InGaN layer 22.

[0101] In the LED shown in FIG. 7 and the light emitting element shownin FIG. 8, light is extracted from the sapphire substrate 20.

[0102] That is, light traveling from the InGaN active layer 22 to thesapphire substrate 20 is output to the outside of the chip via the n-GaNlayer 21 and the sapphire substrate 20 which are transparent to thewavelength of light. On the other hand, light traveling from the InGaNactive layer 22 to the p-AlGaN cladding layer 23 is reflected by the Allayer 34 having high reflectance to the wavelength of light. Thisreflected light is output to the outside of the chip via the n-GaN layer21 and the sapphire substrate 20.

[0103] In the latter case, the light generated in the InGaN layer 22makes a round trip along the path passing through the Ni layer 32 andthe Mo layer 33. Note that the Ni layer 32 and the Mo layer 33 aresufficiently thinned to have a low light scattering coefficient and alow light absorption coefficient.

[0104] A method of manufacturing the LED shown in FIG. 7 and the lightemitting element shown in FIG. 8 will be described below.

[0105] First, as shown in FIG. 9, MO-CVD is used to form an undoped GaNbuffer layer on a sapphire substrate 20 and form an n-GaN layer 21 onthis buffer layer. Subsequently, an InGaN active layer 22 is formed onthe n-GaN layer 21 by MO-CVD or MBE. This InGaN active layer 22 can havean SQW (Single Quantum Well) structure or an MQW (Multiple Quantum Well)structure. In addition, a p-AlGaN cladding layer 23 and a p-GaN layer 24are formed on the InGaN active layer 22 by MO-CVD.

[0106] As shown in FIG. 10, lithography and anisotropic etching such asRIE (Reactive Ion Etching) are used to remove portions of the p-GaNlayer 24, the p-AlGaN cladding layer 23, the InGaN active layer 22, andthe n-GaN layer 21, thereby forming a recess in the edge of the sapphiresubstrate 20. After that, an insulating film 27 is formed on thesurfaces of the p-GaN layer 24, the p-AlGaN cladding layer 23, the InGaNactive layer 22, and the n-GaN layer 21 by CVD.

[0107] This recess can also be formed by isotropic etching such as wetetching, rather than by anisotropic etching such as RIE.

[0108] As shown in FIG. 11, lithography and wet etching are used toremove a portion of the insulating film 27 on the n-GaN layer 21. Afterthat, a Ti layer 28 and an Al layer 29 are formed by vacuum evaporationand lift-off. Also, the structure is annealed in a nitrogen atmosphereat about 600° C. to form an ohmic contact between the n-GaN layer 21 andthe Ti layer 28.

[0109] As shown in FIG. 12, lithography and wet etching are used toremove a portion of the insulating film 27 on the p-GaN layer 24. Afterthat, an Ni layer 32 about 4 nm thick, an Mo layer 33 about 1 nm thick,and an Al layer 34 about 500 nm thick are formed by vacuum evaporationand lift-off.

[0110] If flash annealing is performed at a temperature of 400° C. to780° C. (preferably 450° C.) for 20 sec immediately after the Ni layer32 is formed, an ohmic contact between the p-GaN layer 24 and the Nilayer 32 can be easily formed.

[0111] It is, however, not particularly necessary to perform this flashannealing if no natural oxide film exists in a portion between the Nilayer 32 and the p-GaN layer 24 and if this portion is satisfactorilyclean.

[0112] Subsequently, as shown in FIG. 8, vacuum evaporation and lift-offare used to form Ti layers 30 and 35 about 100 nm thick and Au layers 31and 36 about 1,000 nm thick on the Al layers 29 and 34, respectively. Toimprove adhesion between a plurality of layers forming electrodes 25 and26, flash annealing is performed at a temperature of about 200° C. ormore (favorably about 250° C.) for 20 sec.

[0113] The temperature of this flash annealing is set to be lower thanthat of flash annealing performed in the step shown in FIG. 12, if flashannealing is performed in the step shown in FIG. 12.

[0114] A semiconductor element of the present invention is completed bythe above method.

[0115] The semiconductor element manufactured by the above method ispackaged to form an LED (semiconductor device) of the present invention.

[0116] That is, as shown in FIG. 7, the light emitting element 11 ismounted on the submount 13 having the ohmic electrodes 14-1 and 14-2made of an Au layer about 3 μm thick by flip chip bonding. Consequently,the n-side electrode 25 is connected to the electrode 14-1 by the bump(e.g., AuSn, PbSn, AgSn) 17, and the p-side electrode 26 is connected tothe electrode 14-2 by the bump 17.

[0117] The submount 13 on which the light emitting element 11 is mountedis adhered to the cup type lead frame 12 by using the conductive paste16. In this state, the electrode 14-1 is electrically connected to thecup type lead frame 12. Also, the electrode 14-2 and the lead frame 12are electrically connected by wire bonding. Furthermore, the lightemitting element 11 is covered with the epoxy resin 18.

[0118] If the light emitting element 11 is formed using an n-GaNsubstrate as a conductive substrate, rather than a sapphire substrate,the n-side electrode can also be formed on the back side of this n-GaNsubstrate.

[0119] The LED of the present invention is completed by the abovemethod.

[0120] In the light emitting element, the LED, and the methods offabricating them, the p-side electrode 26 has the Ni layer 32 forforming an ohmic contact with the p-GaN layer 24, the Mo layer 33 havinga barrier function of blocking impurity diffusion, and the Al layer 34having high reflectance to light generated in the element.

[0121] Generally, an ohmic contact to a GaN layer is difficult to formwhen metals such as Al and Ag having high reflectance to visible lightare used. Therefore, the p-side electrode is conventionally composed ofan ohmic layer for forming an ohmic contact and a high-reflection layerfor reflecting light generated in the element.

[0122] When an LED having this electrode structure is continuouslyoperated, however, interdiffusion of metal atoms occurs between theohmic layer and the high-reflection layer owing to the influence ofheat. This raises the forward voltage of the light emitting diode andreadily deteriorates the element. And the electrode sometimes comes off.

[0123] By contrast, in the present invention the barrier layer (e.g., Molayer) 33 made of a high-melting metal is formed between the Ni layer 32as an ohmic layer and the Al layer 34 as a high-reflection layer. Thisbarrier layer prevents interdiffusion of metal atoms between the ohmiclayer and the high-reflection layer. Accordingly, the present inventioncan prevent a rise of the operating voltage of the LED.

[0124] The ohmic layer (Ni layer 32) and the barrier layer (Mo layer 33)are made of materials substantially opaque to light generated in theelement. By decreasing the thicknesses of these layers, the lightreflectance of the high-reflection layer (Al layer 34) can be increased.

[0125]FIG. 13 shows the emission characteristic of the GaN violet typeLED of the present invention.

[0126] This emission characteristic is represented by the relationshipbetween the electric current injected into the LED and its opticaloutput (emission intensity). Referring to FIG. 13, the solid lineindicates the emission characteristic of the GaN violet type LEDaccording to the present invention, and the broken line indicates thatof a conventional LED.

[0127] As shown in FIG. 13, when the same electric current is injectedinto these LEDS, the optical output of the LED according to the presentinvention is about 1.7 times that of the conventional LED. For example,when the electric current injected into the LEDS is 20 mA (voltage 4.3V), the optical output of the conventional LED is about 4.0 mW, whereasthe optical output of the LED of the present invention is about 6.9 mW(emission wavelength λp=450 nm).

[0128] Also, after the LED of the present invention was operated for1,000 hr at room temperature by using a driving current of 20 mA, theoptical output reduced to about 80% of the initial value. This is a verygood result compared to the conventional LED and indicates that thereliability of the LED of the present invention improved.

[0129]FIG. 14 shows the relationship between the thickness of the Molayer as a barrier layer and the light reflectance of the Al layer as ahigh-reflection layer.

[0130] In this relationship, the thickness of the Ni layer as an ohmiclayer is fixed to 4 nm, and the thickness of the Al layer as ahigh-reflection layer is fixed to 100 nm.

[0131]FIG. 15 shows the relationship between the thickness of the Nilayer as an ohmic layer and the Al layer as a high-reflection layer.

[0132] In this relationship, the thickness of the Mo layer as a barrierelectrode is fixed to 1 nm, and the thickness of the Al layer as ahigh-reflection layer is fixed to 100 nm.

[0133] As shown in FIGS. 14 and 15, the light reflectance of the Allayer largely depends upon the thicknesses of the barrier layer and theohmic layer; the smaller the thicknesses of these layers, the higher thelight transmittance. In particular, the thickness of the ohmic layerinto which light generated in the InGaN active layer initially enters ispreferably as small as possible. For example, when this ohmic layer ismade of Ni, its thickness is set to 10 nm or less.

[0134] The ohmic layer can be formed using materials such as Pt, Mg, Zn,Be, Ag, Au, and Ge and compounds consisting primarily of thesematerials, in addition to Ni. Also, the barrier layer can be formedusing materials such as W, Pt, Ni, Ti, Pd and V and compounds consistingprimarily of these materials, in addition to Mo.

[0135] These ohmic layer and barrier layer can be integrated into asingle layer if they are formed using the same material (Ni or Pt).

[0136] In the light emitting element of the present invention, the Tilayer 35 as a barrier layer and the Au layer 36 as an overcoat layer areformed on the Al layer 34 as a high-reflection layer. Commonly, aconductor pattern of Au is written on a submount on which a lightemitting element is to be mounted. A light emitting element is adheredonto this conductor pattern.

[0137] If, however, a high-reflection layer made of Al or Ag is broughtinto direct contact with the Au conductor pattern, a high-resistancelayer may be formed on the bonding surface between them, or the bondingpower between them weakens.

[0138] In the present invention, therefore, an overcoat layer made ofthe same material as the conductor pattern (e.g., Au) on the submount isformed, thereby preventing the generation of a high-resistance layer andincreasing the bonding power between the light emitting element and thesubmount.

[0139] In addition, in the present invention the barrier layer (Ti layer35) made of a high-melting metal is formed between the overcoat layerand the high-reflection layer. Since this barrier layer preventsdiffusion of metal atoms from the overcoat layer to the high-reflectionlayer, the bonding power between the overcoat layer and the conductorpattern can be increased.

[0140] When the conductor pattern and the high-reflection layer are madeof the same material, it is of course unnecessary to form ahigh-melting-material barrier layer between the overcoat layer and thehigh-reflection layer.

[0141] This barrier layer interposed between the overcoat layer and thehigh-reflection layer can be formed using materials such as W, Mo, Pt,Ni, Ti, Pd, and V and compounds consisting primarily of these materials,in addition to Ti.

[0142] Furthermore, in the present invention the light emitting elementis not in direct contact with the lead frame but is mounted on the leadframe via the submount. In this structure, heat generated in the lightemitting element is efficiently radiated via the submount. This canincrease the heat radiation efficiency and improve the reliability ofthe LED.

[0143]FIG. 16 shows a modification of the light emitting element shownin FIG. 8.

[0144] On an n-GaP substrate 40, an n-InGaAlP adhesive layer 41 and ann-InGaAlP cladding layer 42 are formed. On this n-InGaAlP cladding layer42, an InGaAlP active layer 43 is formed. As InGaAlP, a directtransition type band structure is used to obtain red light to greenlight, unlike AlGaAs for which an indirect transition type bandstructure is used to obtain green light.

[0145] On the InGaAlP active layer 43, a p-InGaAlP cladding layer 44 anda p-GaAs contact layer 45 are formed. A p-side electrode 47 is formed onthe p-GaAs contact layer 45, and an n-side electrode 48 is formed on theback side of the n-GaP substrate 40. The surfaces of the n-InGaAlPcladding layer 42, the InGaAlP active layer 43, the p-InGaAlP claddinglayer 44, and the p-GaAs contact layer 45 are covered with an insulatingfilm 46, except for a region where the p-side electrode 47 is formed.

[0146] The p-side electrode 47 includes an AuZn layer 49, an Mo layer50, an Al layer 51, a Ti layer 52, and an Au layer 53. The AuZn layer 49forms an ohmic contact with the p-GaAs contact layer 45. The Mo layer 50is a barrier layer having a function of preventing interdiffusion ofmetal atoms. The Al layer 51 is a high-reflection layer having afunction of reflecting light generated in the element at highreflectance. The Ti layer 52 is a barrier layer having a function ofpreventing interdiffusion of metal atoms. The Au layer 53 is an overcoatlayer for improving the contact with a submount.

[0147] As shown in FIG. 7, this light emitting element shown in FIG. 16is mounted on a submount 13 by flip chip bonding, with the back side ofthe n-GaP substrate 40 facing up.

[0148] In this modification, the n-side electrode 48 is formed on theback side of the n-GaP substrate 40. That is, this n-side electrode 48is formed on the surface different from the surface on which the p-sideelectrode 47 is formed. Hence, the n-side electrode and the lead frameare electrically connected directly by a bonding wire. However, then-side electrode 48 and the p-side electrode 47 can also be formed onthe same surface.

[0149] In this light emitting element as described above, the barrierlayer (e.g., an Mo layer) made of a high-melting metal is formed betweenthe ohmic layer (e.g., an AuZn layer) and the high-reflection layer(e.g., an Al layer). Since this barrier layer prevents interdiffusion ofmetal atoms between the ohmic layer and the high-reflection layer, arise of the operating voltage of the LED can be prevented. Consequently,effects similar to those of the light emitting element shown in FIG. 8can be obtained.

[0150] A method of fabricating the p-side electrode 47 of the lightemitting element according to the present invention is the same as themethod of fabricating the p-side electrode of the light emitting elementshown in FIG. 8, so a detailed description thereof will be omitted.

[0151]FIG. 17 shows the second embodiment of the light emitting elementof the present invention.

[0152] This light emitting element relates to a GaN violet type LED.

[0153] Compared to the light emitting element (FIG. 8) explained in theabove first embodiment, the characteristic feature of the light emittingelement according to this embodiment is the structure of a p-sideelectrode 26.

[0154] An Ni layer 32 as an ohmic layer in contact with a p-GaN layer 24is made up of a plurality of dots (islands) arranged into arrays. An Molayer 33 as a barrier layer is formed on the Ni layer 32 and the p-GaNlayer 24. Accordingly, the p-GaN layer 24 is in contact with both the Nilayer 32 and the Mo layer 32.

[0155] Of light generated in an InGaN active layer 22, a portion oflight heading to a p-AlGaN cladding layer 23 passes through the Ni layer32 and the Mo layer 33 with low scattering and low absorption. Anotherportion of the light heading to the p-AlGaN cladding layer 23 passesonly through the Mo layer 33 without passing through the Ni layer 32.

[0156] In this light emitting element as described above, the ohmiclayer (Ni layer) for forming an ohmic contact with the p-GaN layer doesnot cover the entire surface of the p-side electrode; this ohmic layerpartially covers the p-side electrode as, e.g., a plurality of dots(islands) arranged into arrays. Therefore, in a region where this ohmiclayer exists, an ohmic contact is formed between the p-side electrodeand the p-GaN layer. In a region where the ohmic layer does not exist,only the barrier layer is formed between the p-GaN layer and the Allayer as a high-reflection layer, thereby shortening the distancebetween the two layers.

[0157] In a region where the ohmic layer is absent, therefore, the lighttransmittance can be increased accordingly. As a consequence, the lightextraction efficiency can be increased.

[0158] The ohmic layer can be formed using materials such as Ni, Pt, Mg,Zn, Be, Ag, Au, and Ge and compounds consisting primarily of thesematerials. The barrier layer can be formed using materials such as Mo,W, Pt, Ni, Ti, Pd and V and compounds consisting primarily of thesematerials.

[0159] The ohmic layer and the barrier layer can also be formed usingthe same material (e.g., Ni or Pt).

[0160]FIG. 18 shows a modification of the light emitting element shownin FIG. 17.

[0161] On an n-GaP substrate 40, an n-InGaAlP adhesive layer 41 and ann-InGaAlP cladding layer 42 are formed. On this n-InGaAlP cladding layer42, an InGaAlP active layer 43 is formed.

[0162] On the InGaAlP active layer 43, a p-InGaAlP cladding layer 44 anda p-GaAs contact layer 45 are formed. A p-side electrode 47 is formed onthe p-GaAs contact layer 45, and an n-side electrode 48 is formed on theback side of the n-GaP substrate 40. The surfaces of the n-InGaAlPcladding layer 42, the InGaAlP active layer 43, the p-InGaAlP claddinglayer 44, and the p-GaAs contact layer 45 are covered with an insulatingfilm 46, except for a region where the p-side electrode 47 is formed.

[0163] The p-side electrode 47 includes an AuZn layer 49, an Mo layer50, an Al layer 51, a Ti layer 52, and an Au layer 53. The AuZn layer 49forms an ohmic contact with the p-GaAs contact layer 45. The Mo layer 50is a barrier layer having a function of preventing interdiffusion ofmetal atoms. The Al layer 51 is a high-reflection layer having afunction of reflecting light generated in the element at highreflectance. The Ti layer 52 is a barrier layer having a function ofpreventing interdiffusion of metal atoms. The Au layer 53 is an overcoatlayer for improving the contact with a submount.

[0164] The AuZn layer 49 as an ohmic layer is made up of a plurality ofdots (islands).

[0165] As shown in FIG. 7, this light emitting element shown in FIG. 18is mounted on a submount 13 by flip chip bonding, with the back side ofthe n-GaP substrate 40 facing up.

[0166] In this modification, the n-side electrode 48 is formed on theback side of the n-GaP substrate 40. That is, this n-side electrode 48is formed on the surface different from the surface on which the p-sideelectrode 47 is formed. Hence, the n-side electrode and the lead frameare electrically connected directly by a bonding wire. However, then-side electrode 48 and the p-side electrode 47 can also be formed onthe same surface.

[0167] In this light emitting element as described above, the barrierlayer (e.g., an Mo layer) made of a high-melting metal is formed betweenthe ohmic layer (e.g., an AuZn layer) and the high-reflection layer(e.g., an Al layer). Since this barrier layer prevents interdiffusion ofmetal atoms between the ohmic layer and the high-reflection layer, arise of the operating voltage of the LED can be prevented. Consequently,effects similar to those of the light emitting element shown in FIG. 8can be obtained.

[0168]FIG. 19 shows the third embodiment of the light emitting elementof the present invention.

[0169] On a sapphire substrate 20, an n-GaN layer 21 and a lightemitting layer 55 are formed. For example, this light emitting layer 55includes, as shown in FIG. 8, an InGaN active layer 22 on the n-GaNlayer 21, a p-AlGaN cladding layer 23 on the InGaN active layer 22, anda p-GaN layer 24 on the p-AlGaN cladding layer 23.

[0170] A p-side electrode 26 is formed on the light emitting layer 55.For example, this p-side electrode 26 includes, as shown in FIG. 8, anohmic layer 32, a barrier layer 33, a high-reflection layer 34, abarrier layer 35, and an overcoat layer 36.

[0171] The p-side electrode 26 is placed in a central portion on theupper surface of the light emitting layer 55. Also, an n-side electrode25 is placed at the edge on the upper surface of the n-GaN layer 21 tosurround the light emitting layer 55.

[0172] The LED shown in FIG. 7 is completed by mounting the above lightemitting element on a lead frame by using a submount and covering thelight emitting element with an epoxy resin.

[0173] This light emitting element can achieve the following effects inaddition to the effects of the light emitting elements of theaforementioned first and second embodiments.

[0174] First, since the p-side electrode is positioned in the centralportion of the chip, the light emitting element is readily aligned whenmounted on the submount. This can facilitate the fabrication of the LEDand thereby improve the throughput.

[0175] Second, since the n-side electrode surrounds the light emittinglayer, an electric current flowing from the p-side to the n-sideelectrode is evenly injected into the active layer. Hence, the lightemitting layer can generate light with high efficiency.

[0176]FIG. 20 shows a modification of the light emitting element shownin FIG. 19.

[0177] The characteristic feature of the light emitting element of thisembodiment is that the shape of a light emitting layer 55 is differentfrom that of the light emitting element shown in FIG. 19.

[0178] In the light emitting element shown in FIG. 19, the lightemitting layer 55 is formed in a wide region including a regionimmediately below the p-side electrode 26, and the shape of this lightemitting layer 55 is a square similar to that of the chip. By contrast,in the light emitting element shown in FIG. 20, the light emitting layer55 is formed only in a region immediately below the p-side electrode 26and a narrow region surrounding that region, and the shape of this lightemitting layer 55 is a circle similar to that of the p-side electrode.

[0179] In the light emitting element of this embodiment, the lightemitting region is limited. Therefore, the light emitting element ofthis embodiment can be used as a signal source of an optical fibersystem or in a system required to operate at high speed.

[0180] In the light emitting elements shown in FIGS. 19 and 20, theshapes of the n-side electrode 25, the p-side electrode 26, and thelight emitting element 55 can be variously changed. For example, thep-side electrode 26 can be a square, or the n-side electrode 25, thep-side electrode 26, and the light emitting layer 55 can take shapesother than a circle and square.

[0181] The light emitting element according to the third embodimentdescribed above is applicable to, e.g., a GaN light emitting element,GaAs light emitting element, and GaP light emitting element. This lightemitting element is also applicable to an LED which uses a conductivesubstrate instead of a sapphire substrate.

[0182]FIG. 21 shows the fourth embodiment of the light emitting elementof the present invention.

[0183] The light emitting element of this embodiment is applied to aGaAs light emitting element and GaP light emitting element, andgenerates red light having a wavelength of, e.g., 620 nm.

[0184] On a p-GaP substrate 60, a p-InGaAlP adhesive layer 61 and ap-InAlP cladding layer 62 are formed. An InGaAlP active layer 63 isformed on the p-InAlP cladding layer 62. An n-type InAlP cladding layer64 is formed on the InGaAlP active layer 63, and an n-InGaAlP windowlayer 65 is formed on this n-type InAlP cladding layer 64. Also, ann-GaAs contact layer 66 is formed on the n-InGaAlP window layer 65, andan n-side electrode 67 is formed on the n-GaAs contact layer 66. Inaddition, a p-side electrode 68 and a light reflecting film 69 areformed on the back side of the p-GaP substrate 60.

[0185] A method of manufacturing the light emitting element shown inFIG. 21 will be described below.

[0186] First, as shown in FIG. 22, MO-CVD is used to form an etchingstopper (e.g., InGaP) 71, an n-GaAs contact layer 66 about 0.1 μm thick,an n-In_(0.5)Ga_(0.15)Al_(0.35)P window layer 65 about 0.5 μm thick, andan n-In_(0.5)Al_(0.5)P cladding layer 64 about 1 μm thick in this orderon an n-GaAs substrate 70.

[0187] Subsequently, MO-CVD or MBE is used to form an undopedIn_(0.5)Ga_(0.1)Al_(0.4)P active layer 63 about 0.2 μm thick on then-InAlP cladding layer 64, and form a p-In_(0.5)Al_(0.5)P cladding layer62 about 1 μm thick and a p-In_(0.5)Ga_(0.15)Al_(0.35)P adhesive layer61 about 0.05 μm thick on the undoped InGaAlP active layer 63.

[0188] Examples of the gallium material are triethylgallium (TEG:Ga(C₂H₅)₃) and trimethylgallium (TMG: Ga(CH₃)₃). Examples of thealuminum material are triethylaluminum (TEA: Al(C₂H₅)₃) andtrimethylaluminum (TMA: [Al(CH₃)₃]₂). Examples of the indium materialare triethylindium (TEI: In(C₂H₅)₃) and trimethylindium (TMI: In(CH₃)₃).An example of the phosphorous material is tertiary-butylphosphine (TBP:C₄H₉PH₂).

[0189] As an n-impurity, Si, Te, or the like is used. As a p-impurity,Zn, Be, or the like is used.

[0190] Subsequently, as shown in FIG. 23, a p-GaP substrate 60 about 200μm thick is adhered onto the p-InGaAlP adhesive layer 61 by thermalcontact bonding. Before this adhesion, the adhesion surfaces of thep-InGaAlP adhesive layer 61 and the p-GaP substrate 60 are well cleaned.

[0191] Also, the n-GaAs substrate 70 is removed by etching.

[0192] As shown in FIG. 24, the etching stopper 71 is removed.

[0193] As shown in FIG. 25, the n-GaAs contact layer 66 is patterned byphotolithography and etching.

[0194] After that, an n-side electrode 67 is formed on the n-GaAscontact layer 66, and a p-side electrode 68 and a light reflecting layer(e.g., Au) 69 are formed on the back side of the p-GaP substrate 70. Inthis manner, the light emitting element shown in FIG. 21 is obtained.

[0195] The lamp type LED is completed by mounting the light emittingelement shown in FIG. 21 on a lead frame and covering this lightemitting element with an epoxy resin.

[0196] This LED emits red light when an electric current flowing fromthe p-side to the n-side electrode is injected into the InGaAlP activelayer 63.

[0197] Of the red light having a wavelength of 620 nm generated in theInGaAlP active layer 63, light heading to the n-InAlP cladding layer 64and the n-InGaAlP window layer 65 is directly emitted to the outside ofthe chip. Of the red light having a wavelength of 620 nm generated inthe InGaAlP active layer 63, light heading to the p-GaP substrate 60 istransmitted through the transparent p-GaN substrate 60 and reaches thep-side electrode 68 and the light reflecting layer 69. This light isreflected by the light reflecting layer 69. The reflected light reachesthe n-InAlP cladding layer 64 and the n-InGaAlP window layer 65 and isemitted to the outside of the chip.

[0198] When the light emitting element of this embodiment was mounted ina package having an emission angle of 100 and operated by a drivingcurrent of 20 mA, the optical output rose to 1.2 times (17 cd) that of aconventional light emitting element.

[0199] This light emitting element is applied to a GaAs or GaP lightemitting element having a flip chip structure. Also, the light emittingelement of this embodiment can reduce the loss produced in the alloylayer between the transparent substrate and the electrode, because thelight reflecting layer is formed in a portion of the back side of thetransparent substrate. As a consequence, in a region where the lightreflecting layer is present, light can be efficiently reflected andemitted to the outside of the chip.

[0200] The material of this light reflecting layer is, e.g., Au. This isso because Au has high reflectance to light having a wavelength of 620nm, which is generated in the InGaAlP active layer.

[0201] Table 1 shows the values of reflectance R and thermalconductivity k of metal materials.

[0202] Assume that these metal materials are in contact with the GaPsubstrate, and that the reflectance is a numerical value with respect tolight having a wavelength of 620 nm. Refractive index n of GaP at thiswavelength is 3.325. Assume also that the thermal conductivity is anumerical value at a temperature of 300 K. TABLE 1 METAL REFLECTANCE RTHERMAL CONDUCTIVITY k MATERIAL [%] [W/m · K] Al 77.6 237 Cr 29.1 90.3Co 37.4 99.2 Cu 87.7 398 Au 92.1 315 Hf 13 23 Mo 20.4 138 Ni 37.5 90.5Nb 18 53.7 Os 5.3 87.3 Ag 88.2 427 Ta 20.3 57.5 Ti 25.8 21.9 W 15 178

[0203] The characteristics required for a light reflecting layer arehigh reflectance and high thermal conductivity. In a light emittingelement using InGaAlP, a lowering of the emission efficiency by heat issignificant. Therefore, efficiently radiating heat generated near theactive layer to the outside of the element is important. For thispurpose, as is apparent from Table 1, a material having high reflectanceand high thermal conductivity, e.g., Au, Ag, Cu, or Al, is used as alight reflecting layer of a light emitting element.

[0204] Referring to FIG. 21, the area of the p-side electrode 68 and thearea of the light reflecting layer 69 have the following relationship.That is, when the area of the p-side electrode 68 is made larger thanthat of the light reflecting layer 69, the contact resistance decreases,but the light reflection efficiency lowers; when the area of the lightreflecting layer 69 is made larger than that of the p-side electrode 68,the light reflection efficiency rises, but the contact resistanceincreases.

[0205] In the light emitting element of this embodiment, the area ratioof the p-side electrode 68 to the light reflecting layer 69 is set at,e.g., 1:1. However, this ratio can be appropriately changed inaccordance with the specification of a light emitting element. Forexample, in a light emitting element in which a rise of the contactresistance is of no problem, the area of the light reflecting layer 69is made larger than that of the p-side electrode 68 to raise the lightreflection efficiency.

[0206]FIG. 26 shows a modification of the light emitting element shownin FIG. 21.

[0207] The light emitting element of this modification is characterizedin that the structure of a light reflecting layer is different from thatof the light emitting element shown in FIG. 21.

[0208] A light reflecting layer 69 on the back side of a p-GaP substrate60 is composed of an Si layer 72 and an Al₂O₃ layer 73. The thicknessesof the Si layer 72 and the A1 ₂O₃ layer are so set as to be λ/4n (nindicates the refractive indices of Si and Al₂O₃ with respect to thewavelength of light generated in the active layer) with respect to awavelength λ of light generated in the active layer.

[0209] The Si layer 72 and the Al₂O₃ 73 have a large refractive indexdifference, and the absorption coefficient of the Si layer 72 having ahigh refractive index is small. Therefore, the light reflecting layer 69can achieve high reflectance. However, the Al₂O₃ layer 68 having a lowrefractive index has small thermal conductivity and hence deterioratesthe thermal characteristics of the element.

[0210]FIG. 27 shows the relationship between the electric current andthe optical output of the light emitting element shown in FIG. 21.

[0211] Referring to FIG. 27, line {circle over (1)} corresponds to thelight emitting element shown in FIG. 21; line {circle over (2)}, thelight emitting element shown in FIG. 26; and line {circle over (3)}, aconventional light emitting element.

[0212] According to this relationship, the light emitting element shownin FIG. 21 is most superior in optical output and durability. Theoptical output of the light emitting element shown in FIG. 26 saturateswhen the injection current increases under the influence of low thermalconductivity of Al₂O₃. However, when the injection current is 150 mA orless, the optical output of this light emitting element shown in FIG. 26is higher than that of the conventional light emitting element.

[0213] When the driving current is 20 mA, the characteristic of thelight emitting element shown in FIG. 26 is substantially the same asthat of the light emitting element shown in FIG. 21. Accordingly, it iswell significant to use the light emitting element of this modificationas an LED.

[0214]FIG. 28 shows the fifth embodiment of the light emitting elementof the present invention.

[0215] This embodiment relates to a light emitting element whichgenerates 620-nm red light in a GaAs light emitting element and a GaPlight emitting element.

[0216] A light emitting layer 81 is formed on a portion of an n-GaPsubstrate 80. This light emitting layer 81 includes an n-InGaAlP contactlayer 82, an n-InAlP cladding layer 83, an InGaAlP active layer 84, ap-InAlP cladding layer 85, and a p-InGaAlP contact layer 86.

[0217] An undoped GaP current limiting layer 87 is formed on the otherportion (a region where the light emitting layer 81 is not formed) ofthe n-GaP substrate 80. A p-GaP layer 88 is formed on the light emittinglayer 81 and the undoped GaP current limiting layer 87. A p-sideelectrode 89 is formed on the p-GaP layer 88. An n-side electrode 90 anda light reflecting layer 91 are formed on the back side of the n-GaPsubstrate 80.

[0218] Note that the n-side electrode 90 is positioned immediately belowthe light emitting layer 81.

[0219] A method of manufacturing the light emitting element shown inFIG. 28 will be described below.

[0220] First, as shown in FIG. 29, MO-CVD is used to form ann-In_(0.5)Ga_(0.15)Al_(0.35)P contact layer 82, an n-In_(0.5)Al_(0.5)Pcladding layer 83, an In_(0.5)Ga_(0.1)Al_(0.4)P active layer 84, ap-In_(0.5)Al_(0.5)P cladding layer 85, and ap-In_(0.5)Ga_(0.15)Al_(0.35)P contact layer 86 in this order on ann-GaAs substrate 92. Subsequently, MO-CVD is used to form an undopedGaAs protective layer 93 and an SiO₂ mask layer 94 in this order on thep-In_(0.5)Ga_(0.15)Al_(0.35)P contact layer 86.

[0221] Next, as shown in FIG. 30, the SiO₂ mask layer 94 is patterned byphotolithography and wet etching. This SiO₂ mask layer 94 is used as amask to etch the GaAs protective layer 93, thep-In_(0.5)Ga_(0.15)Al_(0.35)P contact layer 86, the p-In_(0.5)Al_(0.5)Pcladding layer 85, the In_(0.5)Ga_(0.1)Al_(0.4)P active layer 84, then-In_(0.5)Al_(0.5)P cladding layer 83, and then-In_(0.5)Ga_(0.15)Al_(0.35)P contact layer 82 by RIE, thereby forming aridge-shaped light emitting layer 81.

[0222] As shown in FIG. 31, an undoped GaP current limiting layer 87 isformed on the n-GaAs substrate 92 by CVD.

[0223] As shown in FIG. 32, a p-GaP layer 88 is formed on the lightemitting layer 81 and the undoped GaP current limiting layer 87 by CVD.After that, the n-GaAs substrate 92 is entirely etched away to form adevice as shown in FIG. 33.

[0224] As shown in FIG. 34, an n-GaP substrate 80 is bonded to thedevice shown in FIG. 33.

[0225] After that, as shown in FIG. 28, a p-side electrode 89 is formedon the p-GaP layer 88, and an n-side electrode 90 and a light reflectinglayer 91 are formed on the back side of the n-GaP substrate 80.

[0226] Note that the light emitting layer 81 is positioned in a centralportion of the n-GaP substrate 80 (or the chip) and surrounded by theundoped GaP current limiting layer 87.

[0227] Of red light generated in the InGaAlP active layer 84 of thislight emitting element, light traveling to the p-side electrode isemitted to the outside of the chip through the p-InAlP cladding layer85, the p-InGaAlP contact layer 86, and the p-GaP layer 88. Of the redlight generated in the InGaAlP active layer 84, light heading to then-GaP substrate 80 is reflected by the light reflecting layer 91 throughthe n-GaP substrate 80 which is a transparent substrate. This reflectedlight travels to the p-side electrode and is emitted to the outside ofthe chip.

[0228] In this structure, the n-side electrode 90 is positionedimmediately below the InGaAlP active layer 83, and the light reflectinglayer 91 is positioned immediately below the GaP current limiting layer87. That is, light heading to the n-GaP substrate 80 is reflected by thelight reflecting layer 91. Consequently, the reflected light is emittedto the outside of the chip through the GaP current limiting layer 87whose bandgap energy is larger than its emission energy, without passingthrough the light emitting layer 81.

[0229] Since the reflected light does not pass through the lightemitting layer 81, this reflected light is not again absorbed by thelight emitting layer 81. Accordingly, the light emitting element of thisembodiment can achieve sufficiently high light extraction efficiency.For example, when the light emitting element mounted in a package havingan emission angle of 10° is operated with a driving current of 20 mA,the optical output is 1.4 times (about 20 cd) that of a conventionallight emitting element.

[0230] In each of the first, second, and third embodiments of the lightemitting element of the present invention, light generated in the activelayer and traveling to the p-side electrode is reflected by the internalhigh-reflection layer of the p-side electrode, so the entire light isextracted to the outside of the chip from the back side of thesubstrate. In this arrangement, the electrode structure of the p-sideelectrode includes at least an ohmic layer for an ohmic contact, abarrier layer for preventing diffusion of metal impurities, and ahigh-reflection layer for reflecting light generated in the active layerwith high reflectance.

[0231] The barrier layer is made of a high-melting material and preventsinterdiffusion of metal atoms caused by heat between the ohmic layer andthe high-reflection layer. Also, since the thicknesses of the ohmiclayer and barrier layer are made as small as possible, the lightabsorption loss in these ohmic layer and barrier layer can be minimized.Therefore, in this p-side electrode it is possible to realize an ohmiccontact and high light reflectance at the same time and suppress a riseof the operating voltage by heat.

[0232] As a consequence, the light emitting element and thesemiconductor device using the same according to the present inventioncan realize high reliability and high performance. When the ohmic layeris made up of a plurality of dots (islands) arranged into arrays, it ispossible to realize not only an ohmic contact but also high lightextraction efficiency by high reflectance in a region where no ohmiclayer exists, because absorption and loss of light can be reduced by theamount of ohmic layer.

[0233] In each of the fourth and fifth embodiments of the light emittingelement of the present invention, light generated in the active layerand traveling to the substrate is reflected by the light reflectinglayer, so the entire light is extracted to the outside of the chip fromthe surface of the p-GaP layer on the p-side electrode side.

[0234] Also, the electrode and the light reflecting layer arealternately arranged on the back side of the substrate. In a regionwhere the electrode is present, scattering and absorption of lightoccur; in a region where the light reflecting layer is present, light isreflected with high efficiency. Accordingly, the light emitting elementof the present invention can increase the light extraction efficiencyand improve the performance of both the element and the semiconductordevice using the element, compared to conventional light emittingelements.

[0235] Furthermore, the light emitting layer is formed in the shape of aridge and surrounded by a transparent material (undoped GaP). In thisstructure, an electrode is placed immediately below the light emittinglayer, and a light reflecting layer is placed immediately below thetransparent material. Consequently, it is possible to prevent an eventin which light reflected by the light reflecting layer is again absorbedin the light emitting layer, and to increase the light extractionefficiency.

[0236] As has been explained above, the electrode structure of thepresent invention can realize an ohmic contact and high lightreflectance at the same time, and can also prevent interdiffusion ofmetal atoms between a plurality of layers forming the electrode. Thismakes it possible to increase the external quantum efficiency of a lightemitting element, lower the operating voltage, and improve thereliability.

[0237] Additional advantages and modifications will readily occur tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details and representativeembodiments shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit or scope ofthe general inventive concept as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. A light emitting element comprising: a substrate;a light emitting layer formed on said substrate to emit light; and afirst electrode contacting said light emitting layer, wherein said firstelectrode comprises: an ohmic layer in ohmic contact with said lightemitting layer; a first barrier layer formed on said ohmic layer toprevent diffusion of metal atoms; and a light reflecting layer formed onsaid first barrier layer to reflect the light.
 2. The light emittingelement according to claim 1, wherein said substrate is an insulatingsubstrate.
 3. The light emitting element according to claim 1, whereinsaid substrate is a semiconductor substrate.
 4. The light emittingelement according to claim 3, further comprising a second electrodecontacting said semiconductor substrate.
 5. The light emitting elementaccording to claim 1, wherein said light emitting layer comprises alight emitting diode.
 6. The light emitting element according to claim1, wherein said light emitting layer comprises: a first semiconductorlayer of first conductivity type; an active layer formed on said firstsemiconductor layer to emit the light; and a second semiconductor layerof second conductivity type formed on said active layer.
 7. The lightemitting element according to claim 6, wherein said first electrodecontacts said second semiconductor layer.
 8. The light emitting elementaccording to claim 6, further comprising a second electrode contactingsaid first semiconductor layer.
 9. The light emitting element accordingto claim 1, wherein the light is radiated from the back side of saidsubstrate.
 10. The light emitting element according to claim 1, whereinthe light reflected by said light reflecting layer travels to the backside of said substrate.
 11. The light emitting element according toclaim 1, wherein said first barrier layer prevents diffusion of themetal atoms between said ohmic layer and said light reflecting layer.12. The light emitting element according to claim 1, wherein said firstbarrier layer is made of a high-melting metal.
 13. The light emittingelement according to claim 1, wherein the thickness of said firstbarrier layer is not more than 10 nm.
 14. The light emitting elementaccording to claim 1, wherein said ohmic layer is a member of the groupconsisting of a metal, selected from the group consisting of Ni, Pt, Mg,Zn, Be, Ag, Au, and Ge, and an alloy containing the metal.
 15. The lightemitting element according to claim 1, wherein said first barrier layeris a member of the group consisting of a metal, selected from the groupconsisting of W, Mo, Pt, Ni, Ti, Pd, and V, and an alloy containing themetal.
 16. The light emitting element according to claim 1, wherein saidlight reflecting layer is a member of the group consisting of a metal,selected from the group consisting of Al and Ag, and an alloy containingthe metal.
 17. The light emitting element according to claim 1, whereinsaid ohmic layer and said first barrier layer are made of the samematerial.
 18. The light emitting element according to claim 17, whereinthe material of said ohmic layer and said first barrier layer is amember of the group consisting of a metal, selected from the groupconsisting of Ni and Pt, and an alloy containing the metal.
 19. Thelight emitting element according to claim 1, wherein said ohmic layer ismade up of a plurality of islands arranged into arrays.
 20. The lightemitting element according to claim 19, wherein a portion of the lightpasses through said ohmic layer and said first barrier layer and isreflected by said light reflecting layer, and another portion of thelight passes only through said first barrier layer and is reflected bysaid light reflecting layer.
 21. The light emitting element according toclaim 1, wherein said first electrode further comprises: a secondbarrier layer formed on said light reflecting layer; and an overcoatelectrode for mounting formed on said second barrier layer.
 22. Thelight emitting element according to claim 21, wherein said secondbarrier layer is made of a high-melting metal.
 23. The light emittingelement according to claim 1, wherein said first electrode is positionedin a central portion of said light emitting layer.
 24. The lightemitting element according to claim 8, wherein said first electrode ispositioned in a central portion of said light emitting layer, and saidsecond electrode surrounds said first electrode.
 25. The light emittingelement according to claim 4, wherein said second electrode ispositioned on the edge of said semiconductor substrate.
 26. The lightemitting element according to claim 4, wherein said second electrodecomprises a plurality of portions, and said plurality of portions arearranged apart form each other on the back side of said semiconductorsubstrate.
 27. A light emitting element comprising: a substrate; a firstsemiconductor layer of first conductivity type formed on said substrate;an active layer formed on said first semiconductor layer to emit light;a second semiconductor layer of second conductivity type formed on saidactive layer; a first electrode formed in a central portion of saidsecond semiconductor layer and contacting said second semiconductorlayer; and a second electrode formed on the edge of said firstsemiconductor layer and contacting said first semiconductor layer.
 28. Alight emitting element comprising: a substrate; a first semiconductorlayer of first conductivity type formed on said substrate; an activelayer formed on said first semiconductor layer to emit light; a secondsemiconductor layer of second conductivity type formed on said activelayer; a first electrode formed in a central portion of said secondsemiconductor layer and contacting said second semiconductor layer; anda second electrode contacting said first semiconductor layer andsurrounding said first electrode.
 29. A light emitting elementcomprising: a transparent substrate; a first semiconductor layer offirst conductivity type formed on said transparent substrate; an activelayer formed on said first semiconductor layer to emit light; a secondsemiconductor layer of second conductivity type formed on said activelayer; a first electrode contacting said second semiconductor layer; aplurality of second electrodes contacting said transparent substrate;and a plurality of light reflecting layers arranged between saidplurality of second electrodes and contacting said transparentsubstrate.
 30. A light emitting element comprising: a transparentsubstrate; a first semiconductor layer of first conductivity type formedon said transparent substrate; an active layer formed on said firstsemiconductor layer to emit light; a second semiconductor layer ofsecond conductivity type formed on said active layer; a first electrodecontacting said second semiconductor layer; a second electrodecontacting said transparent substrate; and a light reflecting layercontacting said transparent substrate.
 31. The light emitting elementaccording to claim 30, wherein said first semiconductor layer, saidactive layer, and said second semiconductor layer form a ridge-shapedmember in a central portion of said transparent substrate.
 32. The lightemitting element according to claim 31, further comprising: a currentlimiting layer formed on said transparent substrate to cover sidesurfaces of said ridge-shaped member; and a third semiconductor layerformed on said ridge-shaped member and said current limiting layer. 33.The light emitting element according to claim 32, wherein said secondelectrode is positioned immediately below said ridge-shaped member. 34.The light emitting element according to claim 33, wherein said lightreflecting layer surrounds said second electrode.
 35. The light emittingelement according to claim 30, wherein said light reflecting layer ismade of a material selected from the group consisting of a metal and amaterial containing a metal.
 36. The light emitting element according toclaim 30, wherein said light reflecting layer is made of a materialselected from the group consisting of a dielectric substance and amaterial containing a dielectric substance.
 37. The light emittingelement according to claim 30, wherein said light reflecting layercomprises a high-refractive-index layer whose refractive index to thelight is higher than that of said transparent substrate, and alow-refractive-index layer whose refractive index to the light is lowerthan that of said high-refractive-index layer.
 38. The light emittingelement according to claim 30, wherein said first semiconductor layer,said active layer, and said second semiconductor layer are members ofthe group consisting of a compound, selected from the group consistingof InP, GaP, AlP, and GaAs, and a mixed crystal containing the compound.39. A semiconductor device comprising: a lead frame; a submount on saidlead frame; a light emitting element on said submount; and a resincovering said light emitting element, wherein said light emittingelement comprises: a substrate; a first semiconductor layer of firstconductivity type formed on said substrate; an active layer formed onsaid first semiconductor layer to emit light; a second semiconductorlayer of second conductivity type formed on said active layer; a firstelectrode formed in a central portion of said second semiconductor layerand contacting said second semiconductor layer; and a second electrodecontacting said first semiconductor layer and surrounding said firstelectrode, and said first electrode comprises: an ohmic layer in ohmiccontact with said second semiconductor layer; a first barrier layerformed on said ohmic layer to prevent diffusion of metal atoms; and alight reflecting layer formed on said first barrier layer to reflect thelight.
 40. A semiconductor device comprising: a lead frame; a submounton said lead frame; a light emitting element on said submount; and aresin covering said light emitting element, wherein said light emittingelement comprises: a transparent substrate; a first semiconductor layerof first conductivity type formed on said transparent substrate; anactive layer formed on said first semiconductor layer to emit light; asecond semiconductor layer of second conductivity type formed on saidactive layer; a first electrode contacting said second semiconductorlayer; a second electrode contacting said transparent substrate; and alight reflecting layer contacting said transparent substrate.
 41. Amanufacturing method of a light emitting element, comprising the stepsof: forming a first semiconductor layer of first conductivity type on asubstrate; forming an active layer on the first semiconductor layer;forming a second semiconductor layer of second conductivity type on theactive layer; and forming a first electrode contacting the secondsemiconductor layer, wherein the step of forming the first electrodecomprises the steps of: forming an ohmic layer on the secondsemiconductor layer; forming an ohmic contact between the secondsemiconductor layer and the ohmic layer by first annealing; forming afirst barrier layer on the ohmic layer; and forming, on the firstbarrier layer, a light reflecting layer having high reflectance to lightgenerated in the active layer.
 42. The manufacturing method according toclaim 41, further comprising the steps of: forming a second barrierlayer on the light reflecting layer; forming an overcoat electrode formounting on the second barrier layer; and performing second annealing ata temperature lower than that of the first annealing.
 43. Themanufacturing method according to claim 41, wherein said substrate is aninsulating substrate.
 44. The manufacturing method according to claim41, wherein said substrate is a semiconductor substrate.
 45. Themanufacturing method according to claim 44, further comprising the stepof forming a second electrode contacting the semiconductor substrate.46. The manufacturing method according to claim 41, further comprising asecond electrode contacting the first semiconductor layer.
 47. Themanufacturing method according to claim 41, wherein the first barrierlayer prevents diffusion of metal atoms between the ohmic layer and thelight reflecting layer.
 48. The manufacturing method according to claim41, wherein the first barrier layer is made of a high-melting metal. 49.The manufacturing method according to claim 41, wherein the thickness ofthe first barrier layer is not more than 10 nm.
 50. The manufacturingmethod according to claim 41, wherein the ohmic layer is a member of thegroup consisting of a metal, selected from the group consisting of Ni,Pt, Mg, Zn, Be, Ag, Au, and Ge, and an alloy containing the metal. 51.The manufacturing method according to claim 41, wherein the firstbarrier layer is a member of the group consisting of a metal, selectedfrom the group consisting of W, Mo, Pt, Ni, Ti, Pd, and V, and an alloycontaining the metal.
 52. The manufacturing method according to claim41, wherein the light reflecting layer is a member of the groupconsisting of a metal, selected from the group consisting of Al and Ag,and an alloy containing the metal.
 53. The manufacturing methodaccording to claim 41, wherein the ohmic layer and the first barrierlayer are made of the same material.
 54. The manufacturing methodaccording to claim 53, wherein the material of the ohmic layer and thefirst barrier layer is a member of the group consisting of a metal,selected from the group consisting of Ni and Pt, and an alloy containingthe metal.
 55. The manufacturing method according to claim 41, whereinthe ohmic layer is made up of a plurality of islands arranged intoarrays.
 56. The manufacturing method according to claim 42, wherein thesecond barrier layer is made of a high-melting metal.
 57. Themanufacturing method according to claim 41, wherein the first electrodeis positioned in a central portion of the second semiconductor layer.58. The manufacturing method according to claim 46, wherein the firstelectrode is positioned in a central portion of the second semiconductorlayer, and the second electrode surrounds the first electrode.
 59. Themanufacturing method according to claim 45, wherein the second electrodeis positioned on the edge of the semiconductor substrate.
 60. Themanufacturing method according to claim 45, wherein the second electrodecomprises a plurality of portions, which are arranged apart from eachother on the back side of the semiconductor substrate.